The past week has seen one mother, Frances Inglis, jailed for life for murdering a son who was in a persistent vegetative state, and another mother, Kay Gilderdale, acquitted of the attempted murder of her daughter, whose suicide she assisted. The juries in the two cases seem to have been persuaded to their different verdicts by the absence or presence of consent on the part of those killed. Thomas Inglis was unconscious, so had no influence on his mother's actions; Lynn Gilderdale pleaded for her mother's help in ending her life.

The judge in the Gilderdale case went so far as to question whether the prosecution for attempted murder ought to have been brought at all. Campaigners, including Evan Harris of the Liberal Democrats, claim the current law is clumsy and unable to accommodate vastly differing circumstances, with the same blanket law of murder applying to Myra Hindley and the Kay Gilderdales of this world. Harris and pressure groups such as Dignity in Dying would like a new law that would somehow accommodate "mercy killers", but it's a desire I find abhorrent; downright offensive in fact.

Those of a religious persuasion (and I count myself among them) talk about the sacredness of life, non-believers of its inviolability. The Christian view is that life simply isn't ours to take – it's God-given, and his alone to end. The humanist sees the inviolate life as a fundamental right and respect for it as a defining characteristic of a civilised society. The modern law embodies a mixture of both perspectives in its dialogues. But what both religious and humanist strands of thought recognise is that the legal sanctioning of the premeditated termination of human life destroys the principles of sacredness and inviolability. Once "mercy" is introduced as a virtue capable of overriding the absolute duty to preserve life, the sufferer is subject to relativism, fallible human judgment and societal trends and fashions.

One needn't go as far to witness a collective downgrading of regard for life. Since pregnant women have routinely been offered termination on discovery that a foetus has Down's Syndrome, there has been a feeling abroad that choosing to have the child is somehow selfish, self-indulgent even; not fair on the child and definitely not fair on those of us whose taxes will go to support it.

We've developed all manner of intellectual smokescreens to cover our creeping expediency (such as convincing ourselves that it's all about preventing unnecessary suffering), but it would be dishonest not to admit that we've come to pity the carer – the poor person whose life and chances have been sacrificed to the needs of another whose degraded existence will never amount to anything anyway – over the cared-for. In a world where achievement is measured in degrees of personal advancement, years of self-sacrificing care are viewed as wasted.

In the absence of an absolute imperative to preserve life, the idea of a life not worth living becomes very easy to accept. This is precisely the reason why suicide, still a religious offence, was once also a criminal one. It was only partly to do with the victim, and mostly to do with controlling the consequences that would inevitably flow from treating life as expendable. The moment a terminally ill person is granted the legal right to kill herself, the instant corollary is that other feeble forms of life – the late-term foetus with spina bifida, the senile bed-­ridden old woman whose interminable lingering is a torment to her family – are seen as of lesser value than that of a healthy person's and become easier to let go. And once the value of life is placed on a sliding scale, it's a very short hop to saying that there is no right to life beneath a certain threshold, especially when one's continued respiration becomes a heavy burden on others.

But what about the sufferers, don't they have a right to escape their pain? No, not if we believe that life is sacred. We've become so used to the idea that suffering is to be avoided at all costs, that the very notion that we might have to bear it is seen as a violation of some emerging right to a minimum level of comfort. But suffering has a positive purpose. Of course it's tough for the sufferer, but it's only through witnessing the pain and agony of others that we properly develop empathy and compassion. Many of us will suffer at our end, and for years beforehand; but, I would maintain, we have a duty to tolerate our suffering as a sacrifice for the respect our society has learned to accord to life generally: only through coping with and witnessing our suffering will rising generations gain true respect for the miracle of conception and all that follows it.

Of course it's not fair that some suffer and are therefore legally required to suffer more than others, but life isn't fair, and this in itself is a deeply valuable lesson to grasp. In nature, only the lucky ones survive: the very essence of humanity is that we've risen above that and accept many layers of purpose beyond the brute struggle for survival.

The calls for reform following the Inglis and Gilderdale cases perfectly exemplify how emotional responses to individual circumstances can lead directly to moral collapse. When even a high court judge questions whether a mother who injects air into the veins of her daughter should be prosecuted for attempting to murder, we know that the relativism has usurped principle as the basis of our law. Mr Justice Bean has allowed himself to become sentimental; he is the kind of well-meaning individual who, under Evan Harris's regime, might find himself appointed to a panel that would determine whether a euthanasia-seeker had insufficient quality of life to be expected to endure; he is the kind of man who cannot see that suffering and the involuntary self-sacrifice of carers is a necessary part of a truly humane society; he is the kind of man whose weakness in the face of challenging absolute principles is too easily disguised as compassion.

To uphold the sacredness of life, one is also forced to accept that compared with the alternative, to dispatch one in pain is easy, brutal and selfish, even when he or she is pleading for life to end. For it is there, right in the cleft of mortal desperation that the quality of humanity is most tested and respect for life reinforced.